This invention relates generally to radio frequency (RF) amplifiers, and more particularly the invention relates to an RF amplifier having improved linearity and reduced intermodulation distortion.
RF amplifiers that operate outside of the small signal range are inherently nonlinear and thus introduce signal distortion when multiple signals are amplified. Recent requirements in cellular telephone technology have mandated increased capacity which in turn requires broad band-width, high dynamic range linear amplifiers where the output spectrum is free of unwanted intermodulation products. Intermodulation distortion within the RF amplifier pass-band can severely impede proper transmission and reception of cellular signals.
The concept of feed forward amplification to reduce distortion is known and has been successfully applied to RF amplifiers in the past. However, the prior art amplifiers have a number of limitations. U.S. Pat. No. 5,157,345 discloses a feed forward amplifier in which an error signal is introduced in a feed forward arrangement to minimize distortion. The ability of this system to detect phase over appreciable dynamic range is limited by the chosen configuration of RF mixers. Reasonable dynamic range can be obtained only when the local oscillator (LO) port of a mixer is driven to saturation. Theory and practice show that the dynamic range of a mixer not operating at saturation is limited to around 6 dB. Such a mixer does, in normal operation with a saturated LO, generate intermodulation products of its own. Therefore, obtaining appreciable dynamic range and saturation simultaneously in a mixer is mutually exclusive. In the '346 patent, the power at the mixer LO port is directly proportional to the RF input power. Thus, as the RF drive level changes, mixer LO drive changes proportionally. If the LO drive drops more than 6 dB below the optimal LO drive level, the mixer ceases to operate properly.
Further, the '346 amplifier does not compensate for changes in the gain or phase of the error amplifier or the delay line associated therewith. The amplitude and phase adjustments just prior to the error amplifier are preset and an open loop configuration cannot accommodate changes in error amplification parameters due to drive level, temperature, frequency, or ageing.
Other feed forward amplifier patents such as U.S. Pat. No. 4,885,551 disclose feed forward linear amplifier controlled circuitry to scan the prescribed frequency range and detect the amplitude and phase of any undesired signal for subsequent cancellation. While the '551 system offers advantages over other prior art, the system is complex and slow in using a microprocessor and a complex code. Decision-making flow charts require a complex optimization algorithm in software which is slow when compared to well-known and understood closed loop analog implementations.
The present invention is directed to a feed forward RF amplifier having a high dynamic range and low intermodulation distortion through use of closed loop control circuitry.